The manager revealed the midfielder’s status in a pre-match update where he also ruled Liel Abada out of the game for personal reasons. The Israeli winger will observe Yom Kippur, the most significant day in the Jewish calendar, which begins on Wednesday night.
Also missing will be influential forward Kyogo Furuhashi, joined on the injured list by his club captain. McGregor played three times, starting twice, during Scotland’s recent World Cup qualifiers, but was substituted off midway through the second half of the weekend win over Ross County. Now Postecoglou has been dealt further selection issues ahead of the clash with Manuel Pellegrini’s side, who finished sixth in La Liga last season.
The Celtic boss said: "Cal's got a knock – it's not great for us because he’s obviously a very influential player for us. We're going through a period like that at the moment where we're getting some significant outs (absentees) but it's kind of been the state of play since I got here.
"We've been in a constant state of flux and not been able to settle. I see that as an opportunity for us to build resilience through this period. If we can get through these periods without shifting from the kind of team we want to be, then it's going to make us even stronger once we get everyone back."
Right-back Anthony Ralston is available on Thursday night after missing the 3-0 win over the Highlanders at Celtic Park.